For purposes of simplicity this invention is discussed herein particularly as it relates to detonation of explosives in perforating guns for completing wells for production liquid and gaseous materials such as crude oil and natural gas from production formations in the earth. The invention, however, has many other applications within its spirit and scope.
In order to provide an explosive system that is quite safe to handle, it is desirable to eliminate the more sensitive and less reliable components of the explosive system.
Of the well perforating gun firing systems currently available, those employing electrical detonators may be accidentally discharged by stray currents from faulty power circuitry or grounding, radio frequency energy, electromagnetic transients or lightening strikes, all which are common hazards on drilling rigs.
Percussion or stab detonating devices utilize firing pin impact or friction on highly sensitive initiating material; and the firing pin must be prevented from restriking and possibly causing out of zone discharge from accidental jolts and jars in the event that the gun must be removed from the well unfired.
The system of this invention utilizes the abundance of hydrostatic energy typically available in deep well conditions to adiabatically stimulate a mass of gas to an intense heat to reliably initiate detonation of a perforating gun, thereby replacing conventional electrical, percussion, or stab blasting caps that are more vulnerable to accidental discharge during well perforating operations.
The hydraulic/adiabatic system hereof is inherently safe and reliable as it can not be fired at surface or down to a depth where sufficient hydrostatic potential exists. Unlike percussion or stab detonating means that require a metal to explosive friction or impact, only hot gases contact the initiating material and once the adiabatic heat has dissipated the gun may be retrieved from the well more safely whether it has fired or not.
A further and important feature of this invention is that, since the more highly sensitive compounds can be eliminated and the primary high explosive compounds can be detonated directly by means of adiabatically stimulated heat, the handling characteristics of the resulting explosive system is rendered more safe. Further, if the primary high explosive compounds can be eliminated and direct detonation of a secondary high explosive such as RDX can be stimulated adiabatically then the resulting explosive system will be even more safe.
Accordingly, it is a feature of this invention to achieve, by adiabatically induced heat, direct detonation of a secondary explosive composition to thus provide an explosive system that is inherently safe.
An explosive is a chemical composition that when ignited by heat, friction, impact or shock results in a sudden outburst of hot gas.
Explosives may be classified as deflagrating or detonating explosive depending on whether the velocity of decomposition is sub or supersonic. An arbitrary limit dividing deflagration and detonation is 900 meters per second.
Deflagrating or low explosives includes propellants, of which black gun powder is an example, which decompose rapidly with a high heat and pressure at subsonic velocity. As they burn no significant shock waves are produced Smokeless gun powder and ammonium perchlorate used in well bullet and core guns are other examples of powders in this category.
In detonating high explosives the chemical reaction takes place at supersonic velocity, principally within a thin detonation shock wave zone, traveling through the explosive in the order of 4500 to 7000 meters/second.
The detonating explosive category may be subdivided into, primary high explosives which detonate on exposure to relatively weak mechanical shock or flash, and while secondary high explosives are considerably less sensitive they usually require a detonator shock to induce high order detonation Examples of primary high explosives are: lead azide and lead styphnate used in detonators, while examples of the following secondary high explosives: PETN, RDX, HMX, HNS II, and PYX are of interest for well perforating guns employing shaped charges.
Whether the explosive decomposition path of an explosive is deflagration or detonation is dependent on the intensity of the initiating stimulation and confinement pressure as well as the nature of the particular explosive as for example black powder can be made to detonate.
The less sensitive but powerful secondary high explosives such as RDX used in well shaped charges and detonating cords traditionally require a detonator containing primary and secondary explosive to deliver the strong shock required to initiate them to high order.
Detonators used in well applications are of the electric, percussion or stab type according to the method of initiation. The adiabatic heat detonating device of this patent provides a new and safer method of detonating well perforating guns and other explosive or pyrotechnical devices.
Among the various compositions that are capable of being inflamed, ignited or detonated by adiabatically induced heat are the ignition compounds of a common electrical and non-electric blasting caps, deflagration compounds such as gun powder including the well known black powder, primary high order detonation compounds such as lead azide and secondary high order detonating compounds such as RDX. The initiating mixes of common detonators, often lower system temperature ratings AND are typically quite sensitive and therefore involve an element of danger when detonating caps using these compounds are employed in conjunction with powerful high explosive devices such as shaped charge perforating guns in deep oil well conditions. Likewise, primary high explosive compositions such as lead azide and lead styphnate are considered quite sensitive as heat or friction causes them to detonate high order, and thus are dangerous to handle, particularly in an oil well environment. The main body of the explosive charges consists of secondary high explosives, such as RDX, HMX, PYX, HNS, etc. which are extremely powerful but relatively insensitive to heat, shock, impact or friction, and can be handled quite safely but ordinarily require a primary high explosive device for detonation thereof. A typical combination of compounds for use as a high explosive initiating device would include a match compound for initial ignition in an electrical detonator or a friction sensitive compound in a non-electric percussion or a stab detonator that will in turn stimulate a primary detonating explosive compound such as lead azide which detonates and develops a shock wave of sufficient strength to achieve detonation of a secondary explosive such as RDX, etc.
Deflagration devices are often used as instantaneous power sources for developing a force that is utilized to do work. These devices incorporate deflagration compounds such as potassium perchlorate, strontium nitrate and sodium nitrate which, when ignited, inflame slowly relative to the deflagration of gun powder and develop a gas pressure which can be used as a pneumatic source for accomplishing work. For example, in downhole operations for completion of wells, plugs and packers may be set by power charges. Accordingly, it is a feature of this invention to provide a novel method by which power charges may be safely ignited by adiabatically induced.RTM.d heat. It is also a feature of this invention to achieve adiabatic ignition of other combustible liquids or gases, for example, to release energy for doing any suitable work.
After wells have been drilled to the earth formation level of one or more production zones, the well bore intersecting these production zones is most often lined with pipe, typically referred to as well casing. The well casing is cemented in place within the well bore to thus establish a substantially integral relationship between the casing and the formation to thus provide a seal between the casing and the formation and to assure that the casing remains properly in place in the well bore for the extended life of the well being produced. After the casing has been installed, it is necessary to perforate the casing to thus establish communication between the well and the formation to be produced. These perforated intervals may be isolated by means of packers which establish a seal between the casing and production tubing that extends within the casing from the level of the production formation to the surface.